1. Field of the Invention
This invention relates to a hybrid vehicle and, more particularly, to a hybrid vehicle driven by a motor and an internal-combustion engine.
2. Description of the Related Art
Hybrid vehicles which use both a fuel-fed conventional engine and a motor using clean electric energy, have been developed. Various types of hybrid vehicles have been proposed, such as hybrid vehicles of the series type in which electric power generated by driving a generator by the output rotation of an engine, is changed into direct current to charge to a battery and electric power from the battery is changed into alternating current to drive a motor. Another hybrid vehicle is the parallel type in which the engine and the motor are connected to each other through a clutch, the motor is driven when starting, the clutch is subsequently engaged to drive the engine and the output of the motor is added to that of the engine when accelerating. Yet another type of hybrid vehicle is a combination of the series type and the parallel type.
In a hybrid vehicle, as in a more conventional vehicle using only an engine, the engine operates in an idling state even though the vehicle is halted. Therefore, fuel efficiency is reduced by consuming fuel when the vehicle is not moving. Further, the idling of the engine produces noise due to the idling and exhaust of gas. Consequently, an engine interruption system has been proposed in which the engine is operated only when the engine is needed for driving and, at other times, is turned off in order that the amount of exhaust gas amount is decreased and fuel consumption is improved. The foregoing suggests that such an engine interruption system might be employed for the hybrid vehicle of the parallel type. In the engine interruption system, the operation of the engine is halted when the vehicle is halted, the engine is started again when the accelerator pedal is operated, and then the vehicle is started in motion.
However, the engine interruption system inherently creates a number of problems. Firstly, the clutch must release or connect whenever the engine is stopped or started, with the result that the load on the clutch is increased. Secondly, the frequency of use of the starter is increased because the starter must engage whenever the engine is started, so that durability of the starter becomes a problem. Thirdly, the driving comfort suffers due to the timelag between when the engine is started again after the accelerator pedal is operated and the torque change when the engine output is transferred to the output shaft. If the clutch is immediately connected in order to decrease the timelag, the load on the clutch is further increased.